Bannerman's Castle is on Pollepel Island, in the Hudson River just north of Cold Spring. It was built by a Scotsman in the early days of the 20th century as a storage place for munitions. Unfortunately, an explosion early on, a fire, and time has done its part to turn a castle into ruins, but there are still several walls standing, and the site remains breathtaking. More recently the family donated the island to the Hudson Highlands State Park, and currently there is an active community of volunteers working to stabilize the remaining structures. Occasional tours are available led by them http://bannermancastle.org. The island is visible from an overlook reached from route 9d (park by Little Stony Point or by hiking up Breakneck or Bull Hill. In the summer, many people rent kayaks in Cold Spring, and paddle around the island. Its really fun to go around and study the architecture from the water. Check the tide tables before launching on your own- fighting the current and tides is a lot of work, but with good timing the paddle is only a couple hours. This is a wonderful piece of Hudson Valley history and well worth a visit, coupled with a hike or lunch in Cold Spring.